On Wed, Jul 23, 2014 at 08:03:47AM +0100, AKASHI Takahiro wrote:
On 07/23/2014 05:15 AM, Kees Cook wrote:
On Tue, Jul 22, 2014 at 2:14 AM, AKASHI Takahiro
<takahiro.akashi@xxxxxxxxxx> wrote:
asmlinkage int syscall_trace_enter(struct pt_regs *regs)
{
+ unsigned long saved_x0, saved_x8;
+
+ saved_x0 = regs->regs[0];
+ saved_x8 = regs->regs[8];
+
if (test_thread_flag(TIF_SYSCALL_TRACE))
tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
+ regs->syscallno = regs->regs[8];
+ if ((long)regs->syscallno == ~0UL) { /* skip this syscall */
+ regs->regs[8] = saved_x8;
+ if (regs->regs[0] == saved_x0) /* not changed by user */
+ regs->regs[0] = -ENOSYS;
I'm not sure this is right compared to other architectures. Generally
when a tracer performs a syscall skip, it's up to them to also adjust
the return value. They may want to be faking a syscall, and what if
the value they want to return happens to be what x0 was going into the
tracer? It would have no way to avoid this -ENOSYS case. I think
things are fine without this test.
Yeah, I know this issue, but was not sure that setting a return value
is mandatory. (x86 seems to return -ENOSYS by default if not explicitly
specified.)
Is "fake a system call" a more appropriate word than "skip"?
I will defer to Will.
I agree with Kees -- iirc, I only suggested restoring x8.
Will--